A multi-conductor wireline with integral electrical conductors is often used in the oil and gas industry to physically transport a logging tool downhole and to transmit data between a downhole tool and a surface station. Generally, most of the data transfer involves sending data from the tool to recording equipment located on the surface, but certain applications require sending data down to the tool from the surface. The wireline may have from one to seven or more separate electrical conductors, typically composed of copper, depending on the requirements of the application. These conductors have electrical insulation and are wrapped in helical strands of high-strength armor, which are typically made of steel. In the particular case of seven-strand wireline cable, there is a single insulated inner conductor wrapped by a helix of six insulated conductors, overlaid by two layers of inner and outer armor strands that are wrapped in opposing directions. In many applications, each conductor is assigned to a single dedicated analog or digital signal. Typical dimensions are about 0.04-inch (1 mm) for the diameter of each conductor and up to 30,000 ft (9146 m) for the length of the wireline, although variations from these typical values are common.
Electrically, the wireline cable acts as a distributed low-pass filter with frequency-dependent coupling, displaying a complex impedance characteristic with mutual impedance between conductors. A signal transmitted on any given channel (consisting of a voltage difference between two terminals) is subjected to attenuation (or reduction in signal amplitude) between the input signal and the output signal, This attenuation typically increases with frequency and is a result of “self-impedance.” Self impedance is the resistance, capacitance and inductance of an individual conductor. Also, transmitting a signal on one set of conductive paths generates a corresponding signal on other paths by the existence of “mutual impedance” that results from capacitive and inductive coupling between the conductors. This coupling is known as crosstalk. Typically, for transmission systems such as a conventional multi-conductor wireline, the amplitude of the induced crosstalk signal increases with the frequency of the input signal.
As more fully described in U.S. Pat. Nos. 5,747,750 and 5,917,160 (Bailey, et al), a method has been developed to extend the usefulness of this conventional wireline to a greater number of data signals and to increase the maximum frequency bandwidth of these signals. In particular, this method was designed for use with a Triaxial Borehole Seismic (TABS) downhole logging tool to record microseismic acoustic data that are emitted by formations after fluids injection at fracturing rates. The TABS fracture mapping technology uses a relatively inexpensive, reusable, clamped logging tool and specialized data analysis methods to locate microseismic events. The induced fracture geometry may be inferred from maps composed of collections of such event source locations. Fracture azimuth, length, height, and orientation (vertical or horizontal) may be estimated from this data. Such data may be critical for optimizing the field development plan of a fracture stimulated reservoir.
Modern logging tools (e.g., TABS) are capable of producing large quantities of information characterizing subsurface formations. This information must be communicated or transmitted to the surface so that it may be utilized (e.g., to locate underground oil deposits or identify fracture propagation geometry). Data telemetry is no simple task since the number of signals to be transmitted may exceed the number of conductors, hereinafter referred to as “telemetric conductors,” available in the wireline. Moreover, signal attenuation and frequency-dependent crosstalk commonly occur at higher frequencies in the data telemetry process, limiting the signal bandwidth.
Multi-channel frequency-modulated (FM) telemetry as described in the Bailey et al. patents was developed for the first generation of the TABS tool. Signal transmission by frequency modulation is preferred to amplitude modulation because the signal amplitude is significantly attenuated along the wireline, but the signal frequencies are preserved. At the surface data recorder, each frequency can be selected and demodulated to recover a facsimile of the original signal. In the prior art TABS system, seven channels of seismic data are transmitted on FM signals over four conductors, leaving three conductors for other uses. Data that is transmitted on these wire pairs have a bandwidth that begins to roll off significantly at 20 kHz (depending on the specific wireline configuration), limiting signal detectability at higher frequencies. Appropriate frequency spacing between channels is required for proper signal separation, and the overall design constraints force the use of lower frequency carrier channels that impose an overall bandwidth limit of about 500 Hz on the modulating and recovered signals.
U.S. Pat. No. 4,796,026 (Hammond) proposes a piezoelectric transducer for noise logging applications. Downhole circuitry is used to convert the noise signal amplitude to a variable pulse rate, similar to a voltage-to-frequency conversion process. In this reference, there is no discussion of transmission of downhole signals to the surface over multiple conductors.
In U.S. Pat. No. 3,514,750 (Pritchett), a multiple conductor encoding scheme for transmission of digitized signals is described. To mitigate the effects of wireline impedance coupling, signals of opposite polarity are transmitted simultaneously on alternate conductors. Encoding methods for 3, 4, 6, and 7 conductors are disclosed.
U.S. Pat. No. 5,062,084 (Schoepf) specifies a system for operating a downhole-digitizing acoustic tool on seven conductor wireline. Two conductors provide power, and the remaining cables provide digital data telemetry, including downlink communication in addition to the uplink data transfer. The system has distinct recording and transmitting modes. Therefore, Schoepfs downhole digitizing acoustic tool cannot provide full real-time data telemetry.
Finally, U.S. Pat. No. 4,646,083 (Woods) describes a system by which an analog signal is transmitted on a FM channel over a pair of wires. The frequency is not defined in the claims, but a value of 15 kHz is cited in the preamble. This signal is the summation of two square waves with frequencies that are proportional to two DC values, such as temperature and pressure. This reference does not consider the use of additional conductors or the assignment of conductors to particular signals to mitigate wireline impedance.
Previous attempts to address wireline telemetry have been based on the concepts of digital encoding, transmission of low frequency signals, and avoiding simultaneous transmission of multiple signals. The prior art does not include methods to transmit analog or digital signals at frequencies in excess of 20 kHz on oilfield wirelines of practical length and construction.
Accordingly, there is a need for an improved data telemetry system whereby analog or digital signals of up to 100 kHz can be transmitted uphole in a manner such that signal attenuation and crosstalk are reduced significantly. Preferably, this transmitted signal can contain multiple FM carrier bands in order to transmit multiple channels of analog or digital data that individually have bandwidths up to 3 kHz or higher. For maximum utility, the system must be capable of transmitting a large number of signals, far in excess of the number of conductor pairs available on the wireline.